TD-SCDMA (time division-synchronized code division multiple access) is one of the three main standards for 3G (the third generation telecommunication) and has an extensive application prospect.
Both a BS (base station) and a UE (user equipment) transmit and receive in nominal carrier frequencies in the TD-SCDMA system. There is a deviation between a practical transmitting frequency and a practical receiving frequency for the level limitation of equipments. TD-SCDMA system requires that the carrier frequency error of a BS is less than 0.05 PPM, and requires that the carrier frequency error of a UE is less than 0.1 PPM. At the BS side, the frequency precision of an oscillator can meet the requirement since the limitation, such as of temperature, volume, power consumption and cost and so on, is low; but at the UE side, the frequency precision of a selected crystal oscillator usually can not meet the requirement for the limitation of various factors. The function of AFC (automatic frequency control) is to correct the carrier frequency error between the BS and the UE to ensure the performance of the subsequent demodulation and decoding.
The existing frequency deviation estimations are performed usually according to the DwPTS (downlink pilot timeslot) and the Midamble in a beacon channel. Its principle is to divide the data into a front part and a rear part, which are then estimated respectively, and the carrier frequency deviation is calculated according to the difference between the phase rotations of the estimation values of the two parts.
Due to the limitation of the short characteristic of the synchronized code, the deviation of sampling locations or the multi-path distribution will cause the existing method to degrade obviously to a biased estimation near the 0 frequency, and this kind of deviation is difficult to solve through AFC control strategy, and only can the approach of trying to maintain the work situation be used to reduce this kind of influence. For example:
(1) realizing more accurate timing to ensure the sampling value location deviation is far less than ¼ chip (a time unit, 1.128 Ms).
(2) using multi-path location estimation technology, such as inter-path interference cancellation, multi-path channel reconfiguration and so on, to impress the negative effect of the multi-path interference.
However, a series of new problems are brought in the system by the introduction of the above technology:
(1) the errors of the above estimation and cancellation technologies themselves bring in new interferences, which reduce the performance and stability.
(2) most of the technologies need a high frequency precision as a premise, and the effect of such estimations and cancellations will decrease along with the reduction of the frequency deviation estimation performance, and will further cause reduction of the frequency deviation estimation performance, thus causing a vicious circle and the system to be not stable enough.
(3) in a bad situation, how to realize the above technologies is a difficulty itself, which increases the realization complexity and developing period.